Strong, corrosion resistant alloy

ABSTRACT

Alloys containing special percentages of chromium, nickel, cobalt, carbon, iron, etc., are highly corrosion resistant and in the cold-worked condition, afford tensile strengths well above 150,000 psi.

United States Patent 1191 Snape STRONG, CORROSION RESISTANT ALLOY [75]Inventor: Edwin Snape, Ridgewood, NJ.

[73] Assignee: The International Nickel Company,

Inc., New York, N.Y.

22 Filed: Aug. 25, 1972 [21] Appl. No.: 283,810

52 us. c1. 75/122, 75/128 B, 148/39 511 1111.01. C22c 39/00, C22C 39/20[58] Field of Search 75/128 B, 122

[56] References Cited UNITED STATES PATENTS 2,793,948 5/1957 Wagner75/128 June 11, 1974 6/1957 Angel 148/12 5/1970 Gibson 75/128 B PrimaryExaminer-Hyland Bizot Attorney, Agent, or Firm-Ewan C. MacQueen; RaymondJ. Kenny ABSTRACT 4 Claims, No Drawings 1 STRONG, CORROSION RESISTANTALLOY As is known, there are several base alloys capable of resisting,in various degrees, the ravages of corrosive attack. The conventionalstainless steels, cupronickels and aluminum alloys might be cited asillustrative since they have found extensive commercial use in a varietyof corrosive environments. Their acknowledged virtues notwithstanding,such materials have, nonetheless, been found wanting in respect ofapplications requiring high tensile strengths, e.g., upwards of 150,000pounds per square inch (psi). Though the cupronickels and aluminumalloys are generally not hardenable to this strength plateau, manystainless steels can be sufficiently cold worked to impart such strengthlevels; however, this is too often accompanied by the onset of otherdifficulties.

On the other hand, innumerable high strength alloys on the marketgenerally suffer from the inherent incapability of affording corrosionresistance characteristics of a magnitude comparable to those of, say,the stainless steels. And in terms of the present invention even thestainless steels are considerably less than outstanding in resisting tocorrosive influence of such media as stagnant and low velocity seawater.

There are alloys commercially available, though seemingly few in number,which do afford the type of strength and corrosion resistant propertiesmentioned above. But insofar as 1 am aware, they are, comparativelyspeaking, quite costly and/or have given rise to workabilitydifficulties.

In any-case, it has been found that by alloying such constituents asnickel, chromium, cobalt, magnesium, carbon, iron, etc., in carefullycontrolled percentages, alloys can be produced which are (a) markedlycorrosion resistant to various media, including stagnant seawater andother chlorides, (b) hardenable to tensile strengths as high as 200,000psi, and (c) amenable to both hot and cold working.

Generally speaking, alloys in accordance herewith contain (by weight)about 22 to 40 percent chromium, about to 25 0.04 percent nickel, about12 to 30 percent cobalt, up to 0.2 percent magnesium, aluminum, to to2.5 percent silicon, up to'3 manganese, and the balance essentiallyiron, the iron constituting at least 15 percent of the totalcomposition.

In carrying the invention into practice and in striving for the optimumin terms of corrosion resistance, particularly against low velocityseawater, the chromium content should be at least 24 percent, e.g., 26percent or more. It can be as low as 21 percent or possibly percent butat the sacrifice in corrosion resistance. Advantageously, it does notexceed 32 or 33 percent since hot working difl'ficulties can ensue, thehigher chromium percentages tending to introduce or contribute to theformation of embrittling phases, notably sigma. A chromium range of 24to 32 percent is quite satisfactory. I

Nickel contributes to achieving a desired microstructure. 1t greatlyresists the tendency for hard martensite to form during processing,particularly during cold working. A range of 15 or 16 to 23 percentnickel is particularly beneficial.

To attain the tensile strengths contemplated herein, the alloys shouldbe cold worked. The alloys are basically of an austenitic matrix, but byreason of cold working, relatively thin platelets, virtually parallel inarrangement, are formed and distributed throughout the austeniticmatrix. These platelets appear to be much on the order of deformationtwins." At least a small but efiective amount of these thin plateletsshould be present to enhance tensile strength, e.g., at least 2 or 3percent by volume. In this connection, cobalt is deemed to greatlyinfluence this platelet formation and is thus considered to play asignificant role in the strenghtening process. A cobalt range of 17 to23 or 25 percent is quite effective, although it can be as high as 30percent as above indicated. As a practical matter, the improvementsconferred at higher percentages do not appear to justify the added cost.Above 25 percent cobalt, say, 27 or 28 percent or more, it is considered that epsilon phase may be present.

Carbon by reason of its capacity to form carbides and thus potentiallydetract from corrosion resistance, should advantageously not exceed 0.04percent, particularly in the more aggressive environments. A range offrom 0.001 to 0.03 or 0.04 percent is preferred, although it may lessdesirably be up to 0.1 or 0.15 percent.

With regard to magnesium, it is considered beneficial in respect of hotworkability, especially in respect of edge cracking. Experimental datahave not shown such other constituents as calcium, titanium and aluminumto be as effective in this regard. A retained magnesium levelof fromabout 0.002 or 0.005 percent and up to 0.1 percent or possibly 0.15percent is deemed quite desirable.

As to other elements, aluminum and titanium can be used for addedstrength or other purposes but excessive amounts should be avoided tominimize hot workability problems. A range of 0.02 to 1 or 2 percent ofaluminum and from 0.01 to 0.4 percent titanium can be utilized. Up to2.5 percent silicon can be incorporated for castings, but for wroughtalloys it is of benefit that the silicon content not extend beyond about1.5 or 2 percent, this to obviate promoting edge cracking or otherworking difficulties. Manganese need not exceed 1 or 1.5 percent andshould be held to 0.8 percent or less. A range of 0.1 or 0.2 to 0.8percent is preferred for both silicon and manganese.

Alloys containing from 26 to 33 percent chromium, 17 to 23 percentnickel, 17 to 23 percent cobalt, carbon in an amount up to 0.03 percent,up to 0.1 percent, e.g., 0.005 to 0.05 percent magnesium, up to 1.5percent aluminum, up to 0.3 percent titanium, up to 1.5 percent, e.g.,0.2 to 0.8 percent, each of silicon and manganese, the balance being atleast 20 or 25 percent iron, are deemed particularly advantageous.

In order to give those skilled in the art a better appreciation of theinvention the following data are given.

Employing vacuum induction melting, an alloy containing 31 percentchromium, 21.1 percent nickel, 19.8 percent cobalt, 0.024 percentmagnesium, 0.026 percent carbon, 0.36 percent silicon, 0.48 percentmanganese, balance iron and impurities was prepared using electrolyticnickel, cobalt, manganese and iron, lowcarbon ferrochromium, andspectrographic carbon. The iron, nickel, cobalt and carbon were firstcharged and heated to 2,850F. The carbon boil was allowed to run tocompletion at which point the ferrochromium, manganese and silicon wereadded. The melt was held for about 2 mins. and then deoxidized withmagnesium (added in the form of nickel-magnesium).

values were 153,000 psi, 184,600 psi, 14.5 percent and 65 percent,respectively.

For corrosion testing purposes, a sheet specimen in the cold worked andaged condition was immersed in 10 percent ferric chloride, a mostaggressive corrodent, for about 72 hrs., the solution being maintainedat room temperature. The test solution is often used to simulate longterm alloy behavior in relatively stagnant seawater. Duplicate samples--were used and crevices .were intentionally induced. The weight loss(average) was about 0.702 gram. This is in marked contrast to what wouldbe expected of A181 316, a well known crevice corrosion resistant alloy.Experience has shown that the weight loss would be three times or moreas high for A151 316. Moreover, the crevice corrosion of the alloywithin the invention is superior in comparison with AlSl 316.

Air melting as well as vacuum processing can be used in production ofthe alloys. In this connection an alloy containing 32 percent chromium,19.1 percent nickel, 18.6 percent cobalt, 0.019 percent carbon, 0.39percent silicon, 0.51 percent manganese, and small amounts of magnesiumand calcium (melt was oxidized with Ni-Mg and Ca-Si) balance iron andimpurities, was produced by air melting and then processed to bar. Acold rolled 0.375 inch specimen gave a Y.S. of approximately 170,000psi, an U.T.S. of 198,000 psi, an elongation of 9 percent and areduction in area of 38.5 percent. The specimen underwent a loss of0.662 gram in the above-described Fe C1 test. After aging at 900F thecorresponding tensile properties were as follows: 197,900 psi, 214.4psi, 6 and 29 percent, respectively. In the aged condition, thecorrosion loss was lower, being approximately 0.344 gram.

In terms of general processing conditions, forging and hot rollingpractices can be conducted over the temperature range of 2,050 to2,350F. Temperatures below 2,000F may result in edge cracking. l-lotrolled material is preferably annealed at 2,100F or above prior to coldworking.

The amount of cold working applied will, of course, depend oncomposition and the strength level desired. Generally speaking,sufficient cold work should be used to induce the formation of at leastabout percent, e.g., percent or more, by volume of the thin plateletsabove described. Not more than 25 to 40 percent of the platelets need bepresent. Aging, when used, should be carried out with the temperaturespan of 800 to l,000F.

Alloys contemplated within the subject invention are useful as fastenersparticularly in connection with oxidizing chloric solutions and marineenvironments. Other marinehardware would include parts for pumps,

.valves, shafting, shocks, cleats, wrought fittings, etc.

The alloys can also be used in chemical plant equipmerit and for suchdiverse utility as aerospace, desalinization and undersea miningapplications. Also, they can be produced and used in conventional millforms, including sheet, strip, bar and rod.

In referring to the iron content of the alloys as constituting thebalance or balance essentially, it is to be understood, as will beappreciated by those skilled in the art, that the presence of otherelements is not excluded, such as those commonly present as incidentalelements, e.g., deoxidizing and cleansing constituents, and impuritiesnormally associated therewith in small amounts that do not adverselyaffect the basic characteristics of the alloys. Non essential elementsthat can be present include up to 2 percent each of copper andzirconium, up to 0.05 percent boron and up to 0.05 percent selenium.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be withinthe purview and scope of the invention and appended claims.

1 claim: I

l. A corrosion resistant alloy characterized by a microstructurecomprised of an austenitic matrix throughout which thin platelets aredispersed, the platelets being substantially parallel and present in aneffective amount of at least about 5 percent by volume sufficient to'impart enhanced tensile strength to the matrix, said alloy consistingessentially of from 22 to 33 percent chromium, about 10 percent to 23percent nickel, about 12 percent to 23 percent cobalt, up to about 0.04percent carbon, up to about 0.2 percent magnesium, up to less than 0.5percent titanium, up to about 3 percent aluminum, up to 2.5 percentsilicon, up to 3 percent manganese, and the balance essentially iron,the iron constituting at least 15 percent of the alloy.

2. A corrosion resistant alloy characterized by a microstructurecomprised of an austenitic matrix throughout which thin platelets aredispersed, the platelets being present at least in a small but effectiveamount sufficient to impart enhanced tensile strength to the matrix,said alloy consisting essentially of about 26 to 33 percent chromium,about 17 to 23 percent nickel, about 17 to 23 percent cobalt, about0.001 percent to about 0.03 percent carbon, up to 0.1 percent magnesium,up to 0.4 percent titanium, up to 1.5 percent aluminum, up to 0.8percent silicon, up to'about 0.8 percent manganese and the balance beingessentially iron, the iron constituting at least 15 percent of thealloy.

3. A corrosion resistant alloy characterized by a microstructurecomprised of an austenitic matrix throughout which thin platelets aredispersed, the platelets being present at least in a small but effectiveamount sufiicientto impart enhanced tensile strength to the matrix, saidalloy consisting essentially of from 22 to 40 percent chromium, about'10 to 25 percent nickel, about 12 to 30 percent cobalt, up to about0.04 percent carbon, about 0.002 percent to about 0.15 percentmagnesium, up to less than 0.5 percent titanium, up to about 3 percentaluminum, up to 2.5 percent silicon, up to 3 percent manganese,'and thebalance essentially iron, the iron constituting at least 15 percent ofthe alloy.

6 to about 0.8 manganese, and the balance essentially iron, the ironconstituting at least 15 percent of the alloy.

2. A corrosion resistant alloy characterized by a microstructurecomprised of an austenitic matrix throughout which thin platelets aredispersed, the platelets being present at least in a small but effectiveamount sufficient to impart enhanced tensile strength to the matrix,said alloy consisting essentially of about 26 to 33 percent chromium,about 17 to 23 percent nickel, about 17 to 23 percent cobalt, about0.001 percent to about 0.03 percent carbon, up to 0.1 percent magnesium,up to 0.4 percent titanium, up to 1.5 percent aluminum, up to 0.8percent silicon, up to about 0.8 percent manganese and the balance beingessentially iron, the iron constituting at least 15 percent of thealloy.
 3. A corrosion resistant alloy characterized by a microstructurecomprised of an austenitic matrix throughout which thin platelets aredispersed, the platelets being present at least in a small but effectiveamount sufficient to impart enhanced tensile strength to the matrix,said alloy consisting essentially of from 22 to 40 percent chromium,about 10 to 25 percent nickel, about 12 to 30 percent cobalt, up toabout 0.04 percent carbon, about 0.002 percent to about 0.15 percentmagnesium, up to less than 0.5 percent titanium, up to about 3 percentaluminum, up to 2.5 percent silicon, up to 3 percent manganese, and thebalance essentially iron, the iron constituting at least 15 percent ofthe alloy.
 4. A corrosion resistant alloy consisting essentially ofabout 24 to 33 percent chromium, about 16 to 23 percent nickel, about 17to 23 percent cobalt, about 0.001 to 0.03 percent carbon, up to 0.4percent titanium, up to 1.5 percent aluminum, up to 0.8 percent silicon,up to about 0.8 manganese, and the balance essentially iron, the ironconstituting at least 15 percent of the alloy.